1. Field of the Invention
The present invention relates to a printing form engraving method using electron beams and apparatus by which a raster consisting of cup-shaped depressions, so-called raster cups, are engraved in rapid succession in the surface of a printing form, for example, a rotogravure cylinder, with the influence of an electron beam.
2. Description of the Prior Art
Methods for engraving printing forms with an electron beam are known from the DEAS No. 11 23 561, the DLLP No. 55 965, the DEOS No. 21 11 628 and the DEOS No. 24 58 370 with which raster cups having a great variety of shapes can be produced. The intensity focus position, energy distribution in the beam, movement of the focal point and influencing duration are controlled for producing different cup geometry.
It is particularly the low-volume raster cups which, in rotogravure, reproduce the light image passages which require a high constancy in production. The exact reproducibility of a cup geometry in the short time available (a few .mu.s) is, altogether, one of the most difficult problems in electron beam engraving.
Experience has shown that one of the basic requirements for governing the light gradations requires a gradation-dependent change of the focus position on the surface of the printing form. The DEOS No. 24 58 370 describes such a method in which the focus position is controlled in such a manner that the focal point plane lies closer to the surface given small raster cups than given large raster cups. However, the method described proceeds from the fact that the electron beam deflects the continuous electron beam from the surface of the printing form by a so-called blanking system in the pauses between engraving two successive raster cups.
This method has the disadvantage that the electron beam is subject to certain recovery effects after returning to the surface of a printing form which make the engraving unstable.
The DEOS No. 29 47 444 discloses a method which no longer deflects the beam from the surface of the printing form but, while retaining its direction, only defocuses the beam to such a degree by a dynamic, magnetic lens in the pause between the engraving of two successive cups so that no processing effect occurs on the surface of the printing form. Because of the brevity of the time available, such defocusing and refocusing succeeds only with an additional dynamic focus coil as disclosed in the DEAS No. 27 52 598.
The above-mentioned coil, of course, can also accomplish the gradation-dependent focus swing required for engraving different cups, which is disclosed in the DEOS No. 24 58 370.
For this purpose, four large-volume cups, the narrowest cross section of the electron beam is placed on the surface of the printing form. A nearly rectangular current distribution occurs over the entire spot. For low-volume cups, the focus is placed somewhat above the surface of the printing form. Although the beam now has a far greater diameter at the surface than given focusing in the surface, the power density distribution in the beam at the surface is similar to a Gaussian distribution. Above all, the central "peak" has a higher power density than the rectangular energy distribution. When, given such an energy density distribution, the beam is permitted to influence the surface only briefly, then, as a result of the "peak", a cup with a small diameter and a low depth results in the center of the beam. Obviously, a threshold behavior of the processing operation is thereby exploited.
Practice has shown, however, that the threshold behavior is extremely difficult to maintain in a stable condition. Slight drift phenomena in the operating parameters of an electron beam gun, particularly thermal drifts, make it impossible to keep this threshold behavior stable for the entire time of engraving a rotogravure cylinder and it is precisely in light image passages (small cups) that the human eye is extremely sensitive to slight density fluctuations.